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gazza

Which scope for Galaxies / Planetaries and small objects

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Hi,

in the past I've asked for suggestions regarding scopes so I'll apologise now for doing so again. My previous advice requests were colored by difficulties I've had with scopes that were not working properly, however after finally getting my 8" Quattro to give a flat field over an APSC (should have bought a Vixen R200SS!), a process that took over 2 years and many wasted nights, and also buying an Esprit 100 (LOVE this scope), my short focal length needs are well taken care of. I now want my final scope purchase to be for small objects like planetaries and galaxies. I have a C11, but while it has fantastic optics, and produces superb images of the planets, it also shows a tilted field that no amount of collimation can cure, so Is not suitable for any sort of deep sky imaging (tried multiple reducer/flattener options....


Meteoblue says my average seeing in Geelong is around 1-1.5 arc secs, however I reckon it is worse than that, but assuming 1.5 arc secs, I'll need to image at around .5-.6 arc secs/pixel to capture detail on the mythical best night of the year. I have 2 cameras, an ASI071 Pro and a QHY9S/filter wheel.

A 12" f4 GSO truss newt will give me .82 secs/pixel with my ASI071 Pro

A 10" F8 GSO RC will give me .49 at F8, and .66 at F6

A 12 F8 GSO RC will give me .41 at F8 and .55 at F6

Which scope would you chooserhfor high resolution imaging?

Is going to 12" over 10" really worth it? - more light?

I really like how fast F4 sucks photons, but people seem to indicate that for smaller targets F ration is less important (F ratio myth?)

I was going to buy a 150mm Esprit, but figured it really didn't give me that much over my current scopes, and besides, this would exhaust my hobby money for the rest of my life - on a small pension.

As an outlier, what about a Skywatcher F5 300mm newt - dirt cheap at $AU1400 versus $AU3700 for the 12" RC? My Mesu would easily carry it?

cheers
Gary

Edited by gazza
typo

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F/ratio is not important. What is important is aperture at resolution (and this stands for all types of imaging, not just galaxies and planetary nebulae).

First determine resolution you will work on. I would recommend to go for ~1"/pixel resolution. According to my calculations, this resolution is good for FWHM stars of 1.6". With a large scope, good seeing (1") and very good tracking / guiding (Mesu should be able to provide you with that no problem) you should be able to do ~1.6" FWHM stars with good optics.

None of combinations will give you that resolution natively, but any scope will with use of binning. That is a plus, you can image at native (reduced or not) resolution and decide if you want to bin later (try and see if you still manage to capture same amount of detail in binned vs regular images).

Based on all of this and my experience with RC8", I would go for RC12. It will have benefit of having large flat(ish) field - so no corrector needed (unless very large sensor, but 28mm of ASI071 should be very well corrected and fairly flat), so that is plus over Newt which will need coma corrector - and unless premium one - it will enlarge your stars somewhat (check spot diagram for any given CC).

It will also give you resolutions of 0.41, 0.55, 0.82 and 1.1 - so you will be able to choose depending on your star FWHM resolution that suit you best. RC has a lot of back focus, so there will be room for filter wheel, OAG and then some :D. It also handles stray light better than newtonian.

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1 hour ago, vlaiv said:

Based on all of this and my experience with RC8", I would go for RC12. It will have benefit of having large flat(ish) field - so no corrector needed (unless very large sensor, but 28mm of ASI071 should be very well corrected and fairly flat), so that is plus over Newt which will need coma corrector - and unless premium one - it will enlarge your stars somewhat (check spot diagram for any given CC).

I thought RC telescopes suffered from astigmatism instead of coma inherently as part of their design, does this only form further off axis compared to a newt's coma?

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1 minute ago, pipnina said:

I thought RC telescopes suffered from astigmatism instead of coma inherently as part of their design, does this only form further off axis compared to a newt's coma?

Quite further away. With my 8" F/6 newton it was starting to show ~4mm of axis. With 8" RC I use ASI1600, which has diagonal of almost 22mm and I can't spot astigmatism in corners (if properly collimated) - it does start to show some FC, but it can be handled with careful focusing. So aberration free field is at least x3 bigger (on sensor) - which translates into about x2 in angular units.

Faster newtonians will have even smaller aberration free field.

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17 minutes ago, vlaiv said:

Quite further away. With my 8" F/6 newton it was starting to show ~4mm of axis. With 8" RC I use ASI1600, which has diagonal of almost 22mm and I can't spot astigmatism in corners (if properly collimated) - it does start to show some FC, but it can be handled with careful focusing. So aberration free field is at least x3 bigger (on sensor) - which translates into about x2 in angular units.

Faster newtonians will have even smaller aberration free field.

I'm not quite sure what FC is. But thanks for the explanation :)

Do many astrograph RCs have optional astigmatism correctors for larger sensors, or is that usually solved by differing mirror properties?

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10 minutes ago, pipnina said:

I'm not quite sure what FC is.

"False colour", perhaps?

James

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8 minutes ago, pipnina said:

I'm not quite sure what FC is. But thanks for the explanation :)

Do many astrograph RCs have optional astigmatism correctors for larger sensors, or is that usually solved by differing mirror properties?

FC is field curvature. It makes astigmatism more obvious. There are how ever field flatteners for large sensors for RC, that reduce both field curvature and consequently astigmatism.

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Here is an example of astigmatism and field curvature both native and corrected (CCF 250mm F/8 RC)

rc-250mm-spot-without-flattener.jpg

With flattener:

rc-250mm-spot-with-flattener.jpg

As you see from first image, stars start to show elongation after 12-13mm, but it can be corrected by careful focusing (somewhere around 2/3 or 3/4 of frame, center stars will be a bit bigger, but corner stars will be nice and round as well). On larger scope corrected field is a bit bigger (same angle, but due to longer focal length - larger field in mm). On the other hand, with suitable field flattener - there is no worries on full frame sensor :D

 

 

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I'm using a Edge HD11 with Atik 414EX camera. you can see my efforts at https://www.astrobin.com/users/astroian/

If you're thinking of this kind of astrophotography may I suggest you take a look at Innovation Foresiight's On Axis Guider. I have one and would throughly recommend them, esp if you go for a SCT.

Cheers, Ian

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12 hours ago, vlaiv said:

Here is an example of astigmatism and field curvature both native and corrected (CCF 250mm F/8 RC)

rc-250mm-spot-without-flattener.jpg

With flattener:

rc-250mm-spot-with-flattener.jpg

As you see from first image, stars start to show elongation after 12-13mm, but it can be corrected by careful focusing (somewhere around 2/3 or 3/4 of frame, center stars will be a bit bigger, but corner stars will be nice and round as well). On larger scope corrected field is a bit bigger (same angle, but due to longer focal length - larger field in mm). On the other hand, with suitable field flattener - there is no worries on full frame sensor :D

 

 

The second image is definitely a large improvement, but I can't help feeling like it wouldn't be great for a 36x36mm sensor. Unless the blue rings show the minimum area that would be noticeable?

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3 minutes ago, pipnina said:

The second image is definitely a large improvement, but I can't help feeling like it wouldn't be great for a 36x36mm sensor. Unless the blue rings show the minimum area that would be noticeable?

I think blue rings show 10nm diameter. For comparison 250mm F/8 scope will have airy disk of 0.01mm. Whether this is intentional or coincidence it seems that in this case circle represents airy disk size (or very close to it). So with field flattener, blur would be minimal compared to resolving power of telescope - not so in case without field flattener, where at 7-8mm distance from optical axis, spot diagram is starting to be as big as airy disk, so deformation should be noticeable (but probably masked by seeing in long exposure), at 12nm and higher (without careful focusing) it is going to be 2-3 times as big as airy disk radius.

Just to make it more meaningful here are two crops at 200% (so 0.5"/pixel) from my RC8" (it is stacked and very stretched)

This is close to center:

image.png.b68d79d25bede96a280d6189364966cb.png

And this would be in corner:

image.png.6be3434f314c2a79a4e4bb10d954387c.png

It is obvious that stars in the corner are a bit bigger and bit out of focus, and you can maybe even start to see some elongation due to astigmatism, but if stars were focused that would not show.

This is without field flattener, showing 2-3mm from axis and 10-11mm of axis. Scope was not fully collimated at the time - this is part of "test shooting" after doing a round of collimation (here I spotted that it can be improved).

 

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Some years back the talk of Celestrons' 9.25" scope was the crème of the crop for solar imaging . Those run about $1200.00 now for an OTA but now Celestron has the Edge for $2300.00 for the OTA but it's supposedly guaranteed flat field to the edge as well as aberration . If I had the funds and wanted it bad enough that would be the OTA i'd go for cause I know Celestrons Optics have always been the best for me . 

https://www.celestron.com/products/edgehd-925-optical-tube-assembly

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I'll take a blunt view and suggest that you have to balance jackassing around with reflectors which don't work or use refractors which do. Yes, of course some reflectors can eventually be persuaded to work. A few even work fairly quickly. :evil4: Some never work and end up collecting dust. 'Some' in that sentence might actually mean 'quite a few.' The only imaging reflector I would ever buy would be a second hand one from somebody I knew, trusted and who had proved that it could work. There are just so many things out to get you!

A good, large, imaging reflector will beat a 6 inch refractor. But having used a good 14 inch reflector belonging to someone else and my my own 5.5 inch refractor (at 0.9"PP) I have no plans to sell the refractor. Life's too short (especially at my age!)

This image is nowhere near at the limit of what our TEC140/Atik 460/Mesu 200 could produce but it was done in a single night in 7 hours. Double that and I think it could be a very respectable image.

Health warning: I really do like refractors!

5ac681528ee48_M51TECHaLRGB7HrsFinspiralcontrastweb.thumb.jpg.0ae91c64e1beba8d119f86109b956ab2.jpg

Olly

Edited by ollypenrice

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I would recommend the Esprit 150 which I use at 0.7 arc seconds/pixel, it does yield very good results on galaxies. Perhaps more importantly, it is maintainence free and just delvers results. To me, this latter aspect is critical since we have so few clear nights in the UK. To see some example images, have a look in my gallery. 

Alan

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1 hour ago, ollypenrice said:

I'll take a blunt view and suggest that you have to balance jackassing around with reflectors which don't work or use refractors which do. Yes, of course some reflectors can eventually be persuaded to work. A few even work fairly quickly. :evil4: Some never work and end up collecting dust. 'Some' in that sentence might actually mean 'quite a few.' The only imaging reflector I would ever buy would be a second hand one from somebody I knew, trusted and who had proved that it could work. There are just so many things out to get you!

A good, large, imaging reflector will beat a 6 inch refractor. But having used a good 14 inch reflector belonging to someone else and my my own 5.5 inch refractor (at 0.9"PP) I have no plans to sell the refractor. Life's too short (especially at my age!)

This image is nowhere near at the limit of what our TEC140/Atik 460/Mesu 200 could produce but it was done in a single night in 7 hours. Double that and I think it could be a very respectable image.

Health warning: I really do like refractors!

 

Olly

Your high standards must be showing if you think that image isn't already respectable!

I think reflectors have their place, I would rather use a 350mm reflector for planetary imaging than a 140mm refractor any day of the week. You simply get more resolution. Although I would also much rather use a 350mm refractor than a 350mm reflector (if nothing else, you don't lose the 20-25% obstructed light imaging reflectors suffer from) but then that would probably be insanely expensive (and possibly quite heavy?)

7 hours sounds like more clear sky than I've had for the last two weeks. Life may be short but it seems to drag when the clouds are around.

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2 hours ago, ollypenrice said:

I'll take a blunt view and suggest that you have to balance jackassing around with reflectors which don't work or use refractors which do. Yes, of course some reflectors can eventually be persuaded to work. A few even work fairly quickly. :evil4: Some never work and end up collecting dust. 'Some' in that sentence might actually mean 'quite a few.' The only imaging reflector I would ever buy would be a second hand one from somebody I knew, trusted and who had proved that it could work. There are just so many things out to get you!

A good, large, imaging reflector will beat a 6 inch refractor. But having used a good 14 inch reflector belonging to someone else and my my own 5.5 inch refractor (at 0.9"PP) I have no plans to sell the refractor. Life's too short (especially at my age!)

This image is nowhere near at the limit of what our TEC140/Atik 460/Mesu 200 could produce but it was done in a single night in 7 hours. Double that and I think it could be a very respectable image.

Health warning: I really do like refractors!

 

Olly

Unfortunately a decent sized refractor is mucho dollars!!! I had tried to swing an Esprit 150 past SWMBO, but found that it was just too much to get by her :-( I was looking for an alternative that I could purchase without suffering undue punishment. Big newts or an RC fit that bill. I also found the secret to getting a flat field in an F4 newt, and am confident I can get them to work now consistently......But if I could., it would be a refractor everytime....it's just that most of us cant swing for what a large one costs.....

cheer

Gary

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Reflectors are a bargain, for sure, provided they deliver. But they're a risk given that, in the present market, many are being priced down below what would allow a consistent standard. I'm quite serious about buying a proven used one. That s what I'd do.

By the way, my TEC 140, and another which lives here, were bought second hand for about 4000 euros each. Still not cheap but probably no longer depreciating either. (I'm trying to furnish you with helpful arguments!)

Olly

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8 hours ago, gazza said:

I also found the secret to getting a flat field in an F4 newt

Do tell!  Personally, I’ve been using a coma corrector, but perhaps this is missing a trick...

...it also adds glass to the imaging train, so you might as well choose a refractor in the first place?

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3 hours ago, ollypenrice said:

Reflectors are a bargain, for sure, provided they deliver. But they're a risk given that, in the present market, many are being priced down below what would allow a consistent standard. I'm quite serious about buying a proven used one. That s what I'd do.

By the way, my TEC 140, and another which lives here, were bought second hand for about 4000 euros each. Still not cheap but probably no longer depreciating either. (I'm trying to furnish you with helpful arguments!)

Olly

Olly, I'll try anything - I love my Esprit 100 - worked perfectly from the very first image. Good refractors rarely come up secondhand in Australia unfortunately. I have not lost hope of the 150, I just have to work out how depressed I have to look before my lady takes pity on me! If I cant put on a sufficiently good performance I'll have to accept an RC or newt....

If you have any arguments that have worked for you please let me know!!

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I'm in a similar position; torn between an EDGE HD and refractor.

One's easy to sneak past the missus; for the other i'd have to wait until she's away on business and get the purchase/ and delivery times coordinated.

pc387

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13 hours ago, AKB said:

Do tell!  Personally, I’ve been using a coma corrector, but perhaps this is missing a trick...

...it also adds glass to the imaging train, so you might as well choose a refractor in the first place?

I had issues with my Quattro in never being able to get an image that was not tilted in at least one corner. I did every mod that is possible - replaced the spider with a cast one from an R200ss, stabilised the mirror cell with extra springs and added screws to minimise movement side to side, and used every collimation tool known to man, and still couldn't get good results. BTW I use the Skywatcher F4 coma corrector, this is exceptionally sensitive to tilt, so I even machined up a special adapter to hold the comma corrector tight and parallel in the focuser, still had issues.

I then had a real think and read everything I could find and discovered that there is only one position that a secondary mirror will reflect a prefectly flat image on the camera, and at F4 it has to be flat to 30 microns, so there is no leeway in this. The problem is how to do this accurately enough. Conventionally this is done using a sight tube, I did this not accurate enough, I then put a webcam in the focuser and tried to get the mirror right using Mir de Collimation. Still not accurate enough (I was surprised at this).

After some thought I came up with this:

Create a template that is the same size and shape as the secondary mirror

Calculate the offset of the secondary (offset is built in to the secondary holder)

Mark the offset on the template, put a small hole at this position

Put the template on the secondary and use a permanent marker to make a dot on the secondary through the hole in the template

Now get two 2" plastic plugs of the type used to plug focusers, and usibg a lathe put a 1mm hole in the centre of one, and a 1.5mm hole in the second (not critical hole size but must be perfectly central)

Now put the one with 1mm hole in the top of the focuser, and the one with the larger hole in the bottom of the focuser.

Sight through these and adjust the secondary until the dot on it is perfectly central in the holes. Using this has eliminated any form of parallax error. You know it will be right when you can rotate the secondary slightly and the dot still stays central in the hole.

Now use a laser to point the secondary to the centre of the primary, but only use the fore/aft screw. Fix side to side pointing errors by rotating the secondary slightly. If you use the side to side screws, you will throw the secondary off centre.

Then use a cheshire to adjust the primary.

You then need to iterate the secondary/primary alignment until there is no movement in either - the any movement of the primary will very slightly throw off the seondary alignment, this is because the primary is adjusted at it's back which is not where the optical surface is. Ie an adjustment of the primary pushes the optical surface slightly to one side. Three iterations usually fixes this.

At this point everything is aligned an my scope is flat right across an APSC chip. 

The seondary should not need adjustment again, and only a slight tweak of the primary using a chehire is needed at the start of a session - takes 30 seconds..

It took a lot longer to write this, than it takes to actually perform. It only took me 2 years to figure this all out :-)

FWIW

cheers

Gary

Edited by gazza
typos
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9 hours ago, gazza said:

It took a lot longer to write this, than it takes to actually perform. It only took me 2 years to figure this all out :-)

Sincere thanks for taking the effort.  I shall read, learn, and inwardly digest this.  

I may be back with a few questions!  :icon_biggrin:

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1 hour ago, AKB said:

I shall read, learn, and inwardly digest this.

You're gonna need some amount of acid. Seriously tho i want my refractor back.:icon_eek:

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I struggled for 18 months with a decent 24 megapixel  DSLR on an 8" OTA on Alt-Az on a wedge and Celestron 0.63 reducer. Long exposure AP is potentially great if you are not constantly fighting clouds, the risk of drizzle and the inevitable foibles of a sub £3,000 mount.  

I almost got sucked into buying a GEM. Then decided that my tracking and wedge alignment was not the challenge as I was getting down to a superb PAE.  I found myself overwhelmed by an obsession with kit instead of having fun with imaging. Set up and accurate polar alignment took too long and my issues were about time, patience and frustration and a struggle with what, with hindsight, was over complicated hardware and software.

So instead of an expensive GEM I bought a combination of Atik Horizon on Hyperstar for my existing Evolution 8". From my first image I have since enjoyed vastly superior images compared to the DSLR. Imaging at F2 means I can get results using stacked exposures of between 2 seconds and 30 seconds as good as I ever did with all night long term AP; and free of field rotation challenges, I require no wedge; no guiding; no polar alignment; no GEM.  The in excess of 4K UHD large sensor Atik Horizon OSC CMOS with a host of features not available in a CCD is as close to 'point and shoot' as it gets and Atik's Infinity EEA software that now offers colour binning is (IMHO) well worth the camera's slightly premium price compared to the equivalent ZWO AS1600. The FOV is immense and <Zoom> is phenomenal. It is so simple. 

So my advice is don't get sucked into any obsession with solutions requiring the complexities of huge optical tubes, expensive mounts and tripods; auto-guiding etc.; and start with the question "what will be the easiest route to getting great images?". That answer has to be investment in a superior camera operating at F2. So how do you then get enough aperture and F2. My conclusion; either R/C Astrograph or by using your current equipment (if Fastar compatible) by embracing Hyperstar. Both work on fairly similar principles. 

Edited by noah4x4

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